Filter for an aquarium



FIG. 1 is a perspective view of a filter for an aquarium showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a left side elevational view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof; and,

FIG. 8 is a rear perspective view thereof. 

The ornamental design for a filter for an aquarium, is shown and described. 